UseConcMarkSweepGC vs UseParallelGC

I'm currently having problems with very long garbage collection times. please see the followig. My current setup is that I'm using a -Xms1g and -Xmx3g. my application is using java 1.4.2. I don't have any garbage collection flags set. by the looks of it, 3gb is not enough and I really have a lot of objects to garbage collect.

question:

should I change my garbage collection algorithm? what should i use? is it better to use -XX:+UseParallelGC or -XX:+UseConcMarkSweepGC

or should i use this combination

-XX:+UseParNewGC -XX:+UseConcMarkSweepGC

the ones occupying the memory are largely reports data and not cache data. also, the machine has 16gb memory and I plan to increase the heap to 8gb.

What are the difference between the two options as I still find it hard to understand. the machine has multiple processors. I can take hits of up to 5 seconds but 30 to 70 seconds is really hard.

Thanks for the help.

  Line 151493: [14/Jan/2012:11:47:48] WARNING ( 8710): CORE3283: stderr: [GC 1632936K->1020739K(2050552K), 1.2462436 secs]
    Line 157710: [14/Jan/2012:11:53:38] WARNING ( 8710): CORE3283: stderr: [GC 1670531K->1058755K(2050552K), 1.1555375 secs]
    Line 163840: [14/Jan/2012:12:00:42] WARNING ( 8710): CORE3283: stderr: [GC 1708547K->1097282K(2050552K), 1.1503118 secs]
    Line 169811: [14/Jan/2012:12:08:02] WARNING ( 8710): CORE3283: stderr: [GC 1747074K->1133764K(2050552K), 1.1017273 secs]
    Line 175879: [14/Jan/2012:12:14:18] WARNING ( 8710): CORE3283: stderr: [GC 1783556K->1173103K(2050552K), 1.2060946 secs]
    Line 176606: [14/Jan/2012:12:15:42] WARNING ( 8710): CORE3283: stderr: [Full GC 1265571K->1124875K(2050552K), 25.0670316 secs]
    Line 184755: [14/Jan/2012:12:25:53] WARNING ( 8710): CORE3283: stderr: [GC 2007435K->1176457K(2784880K), 1.2483770 secs]
    Line 193087: [14/Jan/2012:12:37:09] WARNING ( 8710): CORE3283: stderr: [GC 20